1. Field of Invention
The present invention relates to zeolitic compositions useful in agriculture and horticulture applications.
2. Discussion of the Background
Zeolites are natural minerals that are mined in many parts of the world or synthesized by man. They are characterized by a microporous, crystalline structure consisting of a three-dimensional framework of SiO4 tetrahedra where all four-corner oxygen ions of each tetrahedra are shared with adjacent tetrahedra. Some of the quadrivalent silicon is replaced by trivalent aluminum, giving rise to a deficiency in positive charge. This charge deficiency is balanced by the presence of mono and divalent cations located in the pores.
These cations are highly mobile and can be exchanged for other cationic species. The Si/Al ratio of natural zeolite is in the range of 1-6. Loosely bound water is also present in the pores of natural zeolites and range from 10-20 wt. % of the dehydrated phase. Zeolites have void spaces (cavities or channels) that can host cations, water or other molecules. Cavities in zeolites enable them to screen molecules and sieve cations.
Cation exchange capacity is primarily a function of the degree of Al substitution for Si in the structure: the greater the substitution, the higher is the deficiency of the positive charge and the greater is the number of alkali cations needed to achieve electrical neutrality. However, cation exchange capacity also depends on other factors, such exposure of the zeolite to cations that are easily trapped in the structure but can not be removed easily. The size of the cations also has an effect on cation exchange capacity. The large cavities and entry channel of zeolites are generally filled with water molecules that form hydration spheres around the exchangeable cations.
The physical and chemical properties of natural zeolites of being to capture and immobilize ammonium ions, water, and certain cations has resulted in numerous investigations into developing agricultural and/or horticultural applications for these materials. Thus the following uses of zeolites have been investigated: holding and slow releasing valuable nutrients to plants, mainly ammonium; uses in conjunction with nitrogen (NH4+), potassium (K+), magnesium (Mg2+), calcium (Ca2+) and trace elements; promoting better plant growth by improving the value of fertilizer; prevention of plant burning from over-use of fertilizers by trapping and slowly releasing valuable nutrients; improving the cation exchange capacity of soil resulting in less fertilizer requirements; and improving the water retention of soil.
Attempts to use zeolites in soil amendments have, however, met with mixed results. For example, it has been reported in U.S. Pat. No. 5,900,387, that Japanese farmers have attained significant crop production improvements when zeolites were added to coarse fertilized soils. It has been is reported that experiments at the Department of Agronomy at Colorado State University have shown that relatively high application rates of zeolites restrict leaching losses of NH4+ from fertilizer in the soil, thereby reducing loss of nitrogen from soils as well as neutralizing low pH soils. See (“Agronomic and Horticultural Uses of Zeolites: A Review”, K. A. Barbarick and H. J. Pirila, Zeo Agriculture and Aquaculture, edited by Wilson G. Pond and Frederick A. Mumpton, West View Press, Boulder, Colo., 1984 (International Committee on Natural Zeolites), pp. 93-103; and pp. 113-122, “Use Of Clinoptilolite In Combination With Nitrogen Fertilization To Increase Plant Growth”, H. J. Pirela et al; and pp. 263-271, “Application Of Clinoptilolite To Soil Amended With Municipal Sewerage Sludge”, M. A. Wilson et al; and references cited in these papers.
However, U.S. Pat. No. 5,900,387 also reports that a body of other experiments have yielded quite mixed results, including experiments on clinoptilolite which has been reported as having a high affinity and selectivity for NH4+, experiments on zeolites having plant fertilizing nutrients such as K+ or with NH4+-containing fertilizer components as soil additives. Some successes have been reported with the use of zeolite/phosphate rock as an exchange medium in the fertilizer system in slowing the release of P in soil growing sorghum-sudangrass has been reported (“Exchange Fertilizer Phosphate Rock plus Ammonium Zeolite Effects on Sorghum-Sudangrass”, K. A. Barbarick et al, Soil Sci. Soc. AM. J., 54: pp. 911-916 (1990); also the report of Barbarick et al published by the Department of Agronomy and Agricultural Experiment Station, Colorado State University, “Response of Sorghum-Sudangrass in Soils Amended With Phosphate Rock and NH4-Exchanged Zeolite (clinoptilolite)”; and “Influence of NH4-Exchanged Clinoptilolite On Nutrient Concentrations In Sorghum-Sudangrass”, D. D. Eberl, K. A. Barbarick and T. M. Lai, Natural Zeolites '93 edited by Douglas W. Ming and Frederick A. Mumpton, International Committee on Natural Zeolites, pp. 491-504, 1995). In the last-named article, increases in nutrient uptake in the plant matter were reported with the addition of NH4-clinoptilolite.
NASA, who has been looking to reducing the amount and cost of carrying plant growing media and, in particular water, in space, has reported that particular zeolite crystals are useful because they permit lowering water and fertilizer component requirements. While such zeolite formulation applications are also useful for water or other purifications and for other environmental clean-up and related purposes, in the case of plant growth applications, only those zeolite formulations that do not result in the production of sodium appear to be suitable.
In the JSC Research and Technology 1993 Annual Report (NASA TM104788), a synthetic soil or substrate for plants (for testing on shuttle flights, called “zeoponic plant growth substrates,”) is reported. This material is described as hydrated zeolite crystals containing loosely bonded ions such as K+, Ca++, Mg++, etc. and combined with calcium phosphate material matrix (apatite). The material is reported to slowly release growth nutrient elements (P, S, Zn, Cu, etc. and the above listed K, Ca and Mg, etc.) into the soil or soil solution for plant absorption or up-take.
However, the ever increasing world-wide demand for food and more efficient uses of natural resources, including water and fertilizers, creates a strong demand for further improvements in agricultural and horticultural materials and techniques. There is, accordingly, a strong demand for new zeolitic composition having improved properties in these applications. In particular, there remains a strong need for an inexpensive and plentiful zeolitic composition that requires little or no processing before use or that possesses a high cation exchange capacity (CEC).